A student prepares a solution by mixing 0.100 mol of ethanoic acid, CH COOH
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student prepares a solution by mixing 0.100 mol of ethanoic acid, CH3COOH, with 0.100 mol of sodium ethanoate, CH3COONa, in enough distilled water to make 1.00 L of solution at 25 °C. The acid dissociation constant for ethanoic acid at this temperature is Ka=1.8×10−5. Small portions of 0.010 M hydrochloric acid are then added dropwise to the mixture.
In answering the following, base your reasoning on chemical equilibrium principles rather than numerical calculation unless specifically instructed.
(a) Identify the major chemical species present in the solution before any hydrochloric acid is added and describe their relative concentrations.
(b) Qualitatively explain, in terms of dissociation and re-association reactions, why the pH of the mixture changes only slightly when a small aliquot of the hydrochloric acid is introduced.
(c) Two additional buffer solutions are prepared. Buffer X contains 0.050 mol CH3COOH and 0.200 mol CH3COONa in 1.00 L of solution, whereas Buffer Y contains 0.200 mol CH3COOH and 0.050 mol CH3COONa in 1.00 L of solution. Without performing numerical calculations, predict which buffer (X or Y) will better resist pH change upon addition of a small amount of solid sodium hydroxide. Justify your prediction.